1. Field of the Invention
The present invention relates to laser scanning systems for reading bar code symbols, and more particularly, to a dual motor speed controller for controlling the speed of two scanning motors in a laser scanning device.
2. Discussion of the Prior Art
The increased use of bar code symbols to identify products, particularly in retail business, has resulted in the development of various bar code reading systems and devices. Presently, many bar code reading devices are portable, hand-held laser scanning systems. The users of these devices place a premium on the size, weight, and power requirements for the devices. One such system is a laser scanning bar code reading system as described in U.S. Pat. No. 4,496,831 assigned to the same assignee as the present invention and incorporated by reference herein.
The laser scanning system disclosed in U.S. Pat. No. 4,496,831 includes a portable, hand-held scanning head which may be embodied in various shapes, one such shape being a gunshaped housing. A handle and barrel portion are provided to house the various components of the laser scanning head therein. These components include a miniature light source such as a semiconductor laser diode, a miniature optic train including focusing lenses for directing light generated from the light source across a bar code symbol, and miniature sensing means for detecting reflected light from the bar code symbol being scanned.
In operation, the miniature optic train focuses a light beam generated by the laser diode to impinge upon a scanning means, which is mounted in the light path within the barrel portion of the scanning head. The scanning means sweeps the laser beam across the bar code symbol, and comprises at least one scanning motor for sweeping the beam lengthwise across the symbol, and may comprise a second motor for sweeping the beam widthwise across the symbol. Light reflecting means such as mirrors are mounted on the shafts to direct the beam through the outlet port to the symbol. The sensing means then detects and processes the light reflected off the symbol, and generally comprises photosensitive elements such as semiconductor photodiodes.
The structural aspects of each of the scanning motors are analogous to a simplified stepper motor, except that the scanning motor is controlled by a control means which is operative to cause the shaft of the motor to oscillate first in one circumferential direction over an arc length less than 360.degree. and secondly in the opposite circumferential direction over an arc length less than 360.degree., and thereafter to repeat the aforementioned cycle at a high rate of speed.
The use of two scanning motors permits multi-line and omni-directional scan patterns to be generated for scanning symbols. For instance, if both x-axis and y-axis scanning is provided, a raster-type scan pattern will be generated for scanning the bar code symbol. A Lissajous-type scanning pattern is available if both scanning motors are driven at sinusoidally-varying rates of speed. The size and shape of the pattern can be controlled by adjusting the tilt angles of the mirrors and/or the angular speeds of the scanning motors. Various scan patterns are described, for example, in U.S. Pat. Nos. 4,251,798, 4,387,297 and 4,871,904. These scan patterns may be used to scan more complex symbols such as two-dimensional bar codes or randomly-oriented symbols.
The U.S. Pat. No. 4,709,195 to R. Hellekson, et al. discloses a bar code scanner that utilizes hall effect devices to monitor the speed of the scanning motor. In particular, the output signal from one hall effect switch is used as an input to a motor detector circuit. The motor detector circuit receives a voltage proportional to the frequency (i.e., speed of the motor), compares the derived voltage to a reference voltage representative of a minimum permissible operating speed, and outputs voltage signals to indicate an overspeed or underspeed motor condition. These voltage signals are monitored by a microprocessor so appropriate action may be taken depending upon the state of the motor.
Presently there is a need for a system that can provide for the independent control of the rate of speed of two scanning motors in a laser scanning device. Moreover, a system that is capable of providing flexibility in the laser scanning patterns that can be generated from such devices is highly desirable. It is accordingly an object of the present invention to fulfill the needs mentioned by providing a dual motor speed control circuit having these desired capabilities.
The LS 9000 scanner, in which the invention is to be applied, is a "lamp type" scanner. The LS 9000 scanner will be operated in a continuous on mode. The scanning pattern will be projected down from the lamp onto a tabletop or to the floor. The actual scanning pattern will be a Lissajous-type which will allow omni-directional reading of bar codes.
To generate the Lissajous-type scanning pattern, two counter rotating motors will be used to deflect the laser beam. By changing the ratio of the speeds of the two motors various Lissajous scan patterns will be generated. The present invention provides a cost effective means for controlling motor speeds which in turn allows a number of scan patterns to be generated.